Shada was the unaired story of Season 17 of Doctor Who. It was unreleased for many years, until 1992, when it was finished using linking narration by Tom Baker to replace missing scenes.
On the Think Tank space station, Doctor Skagra uses a spherical device to drain the minds of his colleagues and departs in his spaceship for Earth, leaving an automated quarantine message running.
In Cambridge 1979, Professor Chronotis has a visit from one of his students, Chris Parsons, who leaves with the wrong book. The Doctor and Romana are enjoying a spot of punting. They’re observed by Skagra and distracted by voices from the sphere he’s carrying. They visit the professor.
Chris discovers that the book is written in a completely alien script. He analyses the book with instruments that make it smoke and glow. Chronotis reveals to Romana he is an elderly Time Lord who has retired to Earth and has been living in the same Cambridge rooms for three hundred years. The Doctor asks him why he was summoned by him to Cambridge but the Professor can’t remember. He later recalls he needs the Doctor’s help finding the book.
Skagra steals a car and the driver’s ability to drive. The Professor reveals the missing book is one he brought from Gallifrey. Skagra drives out to a field where his spaceship is concealed, invisible to the human eye. The Professor confesses the book he took was The Worshipful and Ancient Law of Gallifrey, which dates back to the time of Rassilon and has incredible power. Skagra receives word that all is ready from his carrier ship, commanded by a massive Krarg.
The Doctor and Romana search the Professor’s rooms for the book. They discuss Time Lord law. This reminds them of the Time Lord criminal Salyavin, a boyhood hero of the Doctor. The Professor, asked about his contemporary Salyavin, recalls Chris Parsons’ visit and wonders if he has borrowed the book by accident.
Chris and his friend Clare Keightley analyse the book as the Doctor cycles across Cambridge to the lab. Skagra, now wearing contemporary clothes, comes to the Professor’s college seeking him. While Romana looks in the TARDIS for milk so they can have tea, Skagra arrives for the book. He sets the sphere on the Professor, stealing part of his mind. The Doctor meets Clare at the lab, and examines the book.
Romana and K9 find the collapsed Professor as Chris Parsons arrives. Using the TARDIS medical kit, Romana tries to stabilize the Professor’s condition. The Doctor and Clare discover that the book carbon dates as minus twenty thousand years old. Skagra scans his copy of Professor Chronotis’ mind for a trace of the book but finds nothing. The professor beats out a message to Romana on his hearts in Gallifreyan morse code, telling her to beware of the sphere, Skagra and Shada, but dies before he can reveal where the secret is.
Skagra meets the Doctor trying to return the book to the Professor and has a sphere pursue him on a bike through Cambridge. During the chase the book is dislodged from his basket and retrieved by Skagra. Forced to run away, the Doctor is cornered by the sphere. He tries to escape under a gate, but the sphere approaches and begins to steal his mind…
Romana arrives in the TARDIS to rescue the Doctor. They return to Chronotis’ rooms, where Chris reports the professor’s body disappeared. The Doctor decides he needs to speak with Skagra. K9 tracks the sphere to Skagra’s ship. Clare, worried about the danger the book may pose, comes to Chronotis’s rooms to find them empty. The TARDIS materializes in an apparently empty field which holds Skagra’s invisible ship. They enter and Skagra takes them prisoner. He tells them he was interested only in Chronotis’ mind. He tries to force the Doctor to decode the book and sets the sphere on him to take his mind. K9 fails to blast out of the cell Romana, Chris and he are held in. K9 picks up the sphere’s signals and detects the Doctor amongst its voices. Romana is transported from the cell and taken to the TARDIS by Skagra. He forces her to take it.
While searching the Professor’s rooms for the missing book, Clare finds concealed control panels and accidentally triggers an explosion. The Doctor awakens on the ship and explains to its computer how he survived. The sphere took only a copy of his mind.
Wilkin, the college porter comes to the Professor’s rooms. When he opens the door he doesn’t find the room but the swirling time vortex. The Doctor tries to persuade the ship to release his companions and him, but it decides the sphere must haves succeeded in its task and killed the Doctor. To conserve resources it turns off the oxygen supply…
Chris and K9 are transported out of their cell. The ship detects them and reactivates its oxygen supply. The TARDIS arrives at the Krarg carrier ship and Romana sees the Krargs being grown. Another Krarg starts to form on Skagra’s ship. The Doctor boosts the ships power to allow it to cross space quickly.
In Professor Chronotis’ rooms Clare awakes. She is startled by the Professor, dressed in a night shirt. Skagra is unable to translate the book with the Doctor’s mind in the sphere. The Professor explains to Clare that his rooms are his TARDIS and it interfered to save his life.
The Professor decides they must find Skagra to save the book, which is the key to Shada, the Time Lord prison which has been forgotten. The Doctor and Chris are attacked by a Krarg, but K9 holds it off, allowing them to explore the Think Tank complex where they have arrived. They find the aged bodies of Skagra’s former colleagues. Skagra notices that turning the pages of the book influences the TARDIS. He realizes turning the last page of the book will take him to Shada where he will find the Time Lord criminal Salyavin, who is crucial to his plans.
The Doctor uses Chris’ brain power to revive one of the scientists, the neurologist Caldera. He explains how Skagra set up Think Tank to pool the resources of the mind electronically, but when they had completed the sphere he used it to steal their minds. Skagra now intends to use his mind to dominate the whole of humanity, but need Salyavin to accomplish this. K9 loses his fight against the Krarg and is driven into the Think Tank by the massive creature, which now advances on the Doctor…
The Krarg strikes the machinery in the think tank, creating a huge cloud of smoke. This allows the Doctor, K9 and Chris to escape back to Skagra’s ship, leaving just as the Think Tank station explodes. The Ship is persuaded to take the Doctor to Skagra’s home.
While trying to repair the Professor’s ship Clare asks who Salyavin is.
The Professor places the knowledge that Clare needs to repair the TARDIS in her head telepathically. Skagra’s ship takes the Doctor’s ship to the Krarg carrier ship. They are captured and Skagra reveals his plan to take over the universe telepathically, merging them into one mind: his. The Doctor stages an escape with Chris and K9, but Romana is dragged back to the TARDIS by Skagra.
Fleeing down the corridors of the ship the Doctor and company find an old wooden door and go through it. They are in Professor Chronotis’ rooms/TARDIS. The Professor knows that with the book and TARDIS that Skagra can travel to Shada, which is exactly what he does. Skagra searches Shada’s records for Salyavin, the Time Lord criminal.
The Professor’s TARDIS arrives with K9, the Doctor and him. Skagra starts reviving the prisoners as the Doctor arrives, but when they open Salyavin’s cell they find it empty. The Professor admits he is Salyavin: he escaped centuries ago and used his powers to make the Time Lords forget about Shada.
The Sphere attacks the Professor but is destroyed by K9. However it reforms into several smaller spheres, one of which attaches itself to the Professor. He sinks to the floor. The spheres attach themselves to the revived prisoners, bringing them under Skagra’s control. Chris and Clare arrive, but Chris is taken under the control of a sphere. The prisoners and he advance on the Doctor.
K9 fires at the prisoners, driving them back, but he is thrown aside by a Krarg. The Doctor, Romana and Clare grab K9 and flee to the Professor’s TARDIS. Romana reminds the Doctor that his mind is inside Skagra’s machine too. Skagra returns to the TARDIS and tells the former prisoners that they will return to the carrier ship and be distributed through the universe to further his revolution.
The Doctor follows his TARDIS in the Professor’s. He captures it in a force field and has himself placed into the Time Vortex. The Doctor begins crossing to his TARDIS, but his journey appears in vain. An accident occurs in the Professor’s TARDIS, deactivating the force field, throwing the Doctor into the vortex.
The Doctor finds himself in a room in his TARDIS. He starts building a helmet shaped device. The Professor’s TARDIS arrives on the carrier ship as the Doctor reveals himself and struggles for control of the joint mind.
Romana deactivates the Krarg generating equipment, tipping the gas out and using it to destroy the Krargs. Skagra flees to his ship, but is taken prisoner by his ship’s computer, who has now decided to serve the Doctor.
The Doctor promises to return the prisoners to Shada and summon the Time Lords. He returns his and the Professor’s TARDISes to Earth. This confuses Wilkin, who has returned with a policeman to find the room back in its usual place and the Professor taking tea with his guests.